The present invention relates to controls for effecting direction and speed changes in a hydraulic power shift transmission having multiple forward and reverse speeds and more particularly relates to shift control linkages for effecting direction and speed changes in such transmissions.
Various types of shift control linkages are known in the prior art which include a single lever for effecting direction and speed changes in hydraulic power shift transmissions. Examples of such linkages are disclosed in U.S. Pat. Nos. 3,242,758 granted to Harris et al on Mar. 29, 1966, 3,679,018 granted to Luft on July 25, 1972 and 3,853,019 granted to McAdams on Dec. 10, 1974.
The Harris et al and McAdams patents both show single shift levers which are mounted for pivoting sideways along a first path for selecting different drive directions (neutral, reverse or forward) and for being pivoted along separate paths oriented 90.degree. from the first path for selecting various drive speeds. The structure of these linkages which permit the use of a single shift lever are relatively complex and suffer from the disadvantage of being somewhat difficult to manufacture and assemble.
The Luft patent shows a single shift lever mounted for simply pivoting along a single fore-and-aft path to effect both direction and speed changes and accordingly much of the complexity of the aforementioned Harris et al and McAdams shift linkages is avoided. However, the Luft linkage includes a specially configured cam plate coupled to the shift lever for operating direction and speed selector valves for routing air pressure for operating the transmission shift control valves and consequently the Luft linkage and associated air pressure system still is a rather complex system for effecting shifting of the transmission thereof.